BUG #14973: hung queries

The following bug has been logged on the website:

Bug reference:      14973
Logged by:          Dmitry Shalashov
Email address:      skaurus@gmail.com
PostgreSQL version: 10.1
Operating system:   Debian 9
Description:

We stumbled upon queries running for a day or more. They are simple ones, so
that should not be happening. And most of the time it don't - very small
share of these queries ends up like this.

Moreover, these queries couldn't be stopped.

pg_stat_activity says that they all have wait_event_type = IPC, wait_event =
BtreePage, state = active

strace tells that they all inside epoll_wait syscall

grep over ps says that they all are "postgres: bgworker: parallel worker for
PID ..."

Looks like some bug in parallel seq scan maybe?

We are going to disable parallel seq scan and restart our server in like 4
hours from now. I can get more debug if asked before that.